Automatic test equipment for testing the performance of communications systems, radar systems, and other signal producing devices are known. In this regard, it is often necessary to evaluate the spectral purity of a signal produced by a unit under test (UUT) in order to determine if the UUT is operating within the manufacturer's specifications. Specifically, a manufacturer or end user may specify the maximum phase noise which may be present on a signal produced by the UUT. The phase noise of a signal is a measure of the random phase instability of the signal.
The phase noise of a UUT can be measured in a variety of ways. For example, the output signal of the UUT can be applied directly into the input of a spectrum analyzer which will display the power spectral density of the signal and the phase noise will be visible in the display as random noise power in the spectral plot. Alternatively, the phase noise can be measured using a second signal source as a reference. The second signal source outputs a signal which is identical to or better than the expected UUT signal, but in phase quadrature (if phase modulation noise is being tested) to the UUT signal, i.e. the second signal source is at the same frequency as the UUT signal, but is phase shifted by 90 degrees. The UUT and the second signal source are input into a mixer, and, since the two signals have the same carrier frequency, the signals cancel each other out, leaving a signal comprising the combined phase noise of the UUT and the second signal source. In addition, the phase noise may be measured using the Down Converter/Multiple Direct Spectrum Measurement Technique, which is described in U.S. Pat. Nos. 5,337,014 and 5,179,344, the specifications of which are hereby incorporated by reference.
Conventionally, when a UUT signal is analyzed, the technician uses a variety of discrete components including a programmable down converter for translating the input signal into a lower, and more easily analyzed, frequency; a narrow FM tunable synthesizer for generating a reference signal; and a separate spectrum analyzer. Since each of these components has their own unique programming requirements, significant time and effort is often spent programming and integrating these discrete components into an effective phase noise measurement system.